1. Field of the Invention
The present invention relates to pulse-echo ranging systems, and more particularly to radar or laser rangefinders and time domain reflectometers with precision timing.
2. Description of Related Art
High range resolution pulse-echo systems such as wideband pulsed radars, pulsed laser rangefinders, and time domain reflectometers often sweep a timing circuit across a range of delays. The timing circuit controls a receiver sampling gate such that when an echo signal coincides with the temporal location of the sampling gate, a sampled echo signal is obtained. The range of the sampled echo is directly determined from the timing circuit, so highly accurate timing is needed to obtain high accuracy range information.
Precision timing circuits having scale factor accuracies on the order of several tens of picoseconds or better can be realized with a Delay Locked Loop (DLL) such as a "Precision Digital Pulse Phase Generator" as disclosed by McEwan in U.S. Pat. No. 5,563,605, or in copending application, "Phase-Comparator-Less Delay Locked Loop", Ser. No. 09/084,541, by McEwan now U.S. Pat. No. 6,055,287. Alternatively, dual crystal clocks, one for transmit and one for receive, can be employed, where the receive clock is locked to a small offset frequency from the transmit clock, such as 100 Hz, thereby causing a steady phase slip of one complete clock cycle 100 times per second. In the process, the receive sampler timing smoothly sweeps across one complete pulse repetition interval every 10 ms.
While a high degree of scale factor stability and linearity can be achieved with these techniques, timing offsets are difficult to control on the picosecond level since the propagation delay of every element in the transmit and receive path can contribute several hundred picoseconds of temperature dependent error. Thus, it is essentially impossible to obtain a timing accuracy and stability of less than 6.6 picoseconds as needed for sub-mm ranging accuracy.
Prior art methods to address timing offsets include providing a reference reflector close to the radar antenna, so the radar measures the difference between the reference reflection and the target reflection. This differencing process subtracts out the fixed timing offsets in the radar, leaving the range measurement solely dependent on, in principal, the scale factor accuracy of the timing circuit. This technique has been exploited in a time-domain-reflectometer (TDR) "Electronic Multi-purpose Material Level Sensor", U.S. Pat. No. 5,610,611 by McEwan, wherein a "dipstick" launching plate is used as the reference reflector.
The problems with a reference reflector are twofold: 1) a reflector must be added to the radar "scene", which can be very inconvenient, and 2) reflections from the reference reflector sum with target returns whenever the target is close to the reference reflector, thereby creating an erroneous displaced target echo.
A means is needed to obtain an extremely low drift transmit reference sample relative to the received echo sample without placing a reflector in the field.